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SiP12109DB Datasheet(PDF) 10 Page - Vishay Siliconix |
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SiP12109DB Datasheet(HTML) 10 Page - Vishay Siliconix |
10 / 17 page SiP12109 www.vishay.com Vishay Siliconix S13-2479-Rev. A, 16-Dec-13 10 Document Number: 62694 For technical questions, contact: powerictechsupport@vishay.com THIS DOCUMENT IS SUBJECT TO CHANGE WITHOUT NOTICE. THE PRODUCTS DESCRIBED HEREIN AND THIS DOCUMENT ARE SUBJECT TO SPECIFIC DISCLAIMERS, SET FORTH AT www.vishay.com/doc?91000 Fig. 21 - CM-COT Operational Principle The following equation illustrates the relationship between on-time, VIN, VOUT and RON value: Once on-time is set, the pseudo constant frequency is then determined by the following equation: Loop Stability and Compensator Design Due to the nature of current mode control, a simple RC network (type II compensator) is required between COMP and AGND for loop stability and transient response purpose. The general concept of this loop design is to introduce a single zero through the compensator to determine the crossover frequency of overall close loop system. The overall loop can be broken down into following segments. Output feedback divider transfer function HfbZ: Voltage compensator transfer function GCOMP (s): Modulator transfer function Hmod (s): The complete loop transfer function is given by: When: CCOMP = Compensation capacitor RCOMP = Compensation resistor gm = Error amplifier transconductance Rload = Load resistance CO = Output capacitor RDS(on) = LS switch resistance Rfb1 = Feedback resistor connect to LX Rfb2 = Feedback resistor connect to ground RO = Output impedance of error amplifier = 20 M AV1 = Voltage to current gain = 3 Vcurrent vcomp PWM Fixed ON-time VIN 1 TON = RON x K x , where K = 17.5 x 10 -12 is a constant set internally sw = = = D TON VOUT 1 VIN VOUT VIN x RON x K RON x K H fb R fb2 R fb1 x Rfb2 ------------------------------ = G COMP (s) R O x 1 + sC COMPRCOMP 1 + sR OCCOMP ------------------------------------------------------------------------- gm = H mod (s) 1 AV 1 x RDS(on) ----------------------------------- x R load x 1 + sC ORESR 1 + sC ORload -------------------------------------------------------------- = H mod (s) R fb2 R fb1 x Rfb2 ------------------------------ x R O x 1 + sC COMPRCOMP 1 + sR OCCOMP -------------------------------------------------------------------------gm x 1 AV 1 x RDS(on) ----------------------------------- x R load x 1 + sC ORESR 1 + sC ORload -------------------------------------------------------------- = |
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